Sputtering, alternatively called physical vapor deposition (PVD), is the favored technique for depositing materials, particularly metals and metal-based materials, in the fabrication of semiconductor integrated circuits. Sputtering has a high deposition rate and, in most cases, uses relatively simple and inexpensive fabrication equipment and relatively inexpensive material precursors, targets in the case of PVD. The usual type of sputtering used in commercial applications is DC magnetron sputtering, which is limited to the sputtering of metallic target. Sputtering is widely used for the deposition of aluminum (Al) to form metallization levels in semiconductor integrated circuits. More recently, copper deposition by PVD has been developed. However, sputtering is applicable to a wider range of materials useful in the fabrication of semiconductor integrated circuits. Reactive sputtering is well known in which a target of a metal, such as titanium or tantalum, is sputtered in the presence of a reactive gas in the plasma, most typically nitrogen. Thereby, the sputtered metal atoms react with the reactive gas to deposit a metal compound on the wafer, most particularly, a metal nitride, such as titanium nitride using a titanium target in a nitrogen ambient or tantalum nitride using a tantalum target in a nitrogen ambient.
Sputtering of yet other metals is also important in the fabrication of semiconductor integrated circuits. In a conventional process to contact a metallization of, for example, aluminum to silicon, a thin layer of titanium, typically of thickness less than 15 mm, is deposited over narrow source and drain portions of a silicon substrate which have previously been implanted with p-type or n-type dopants such as boron or phosphorous. The wafer is then annealed, such as by rapid thermal processing (RTP), to cause the titanium and silicon near their interface to diffuse together to form a silicide, in this case titanium suicide. The silicide promotes the adhesion of metallization afterwards deposited over the silicide and also provides a more ohmic contact between the metallization and the semiconducting silicon.
Titanium-based silicides, usually in the form of TiSi.sub.2, however, have some limitations. The temperature required to react titanium with silicon to form the suicide is relatively high, in the range of 600 to 900.degree. C. dependent upon the characteristics required. This is a relatively high temperature and may deleteriously cause the implanted dopants to diffuse away from the intended region of the junction. Furthermore, titanium silicide has shown a tendency to increase its resistivity as it is deposited into increasingly narrow source and drain widths. An acceptably low sheet resistance of 4.OMEGA./.quadrature. is obtained at a line width of 1 .mu.m, but the sheet resistance increases to about 20.OMEGA./.quadrature. at 0.3 .mu.m.
For these reasons, alternative suicides have been considered. Both cobalt and nickel suicides offer much promise. Their siliciding temperatures are lower by 50 to 200.degree. C. They introduce less stress during the siliciding process. Their dependence of resistivity upon line width is negligible. Nickel silicide (NiSi) has some potential advantages in its low resistivity, low stress, and low consumption of silicon during siliciding. However, it has not been favored because it is stable only to about 750.degree. C., versus 900.degree. for TiSi.sub.2 and 1000.degree. with CoSi.sub.2. For these reasons, cobalt silicide has been more extensively investigated as a replacement for titanium silicide.
Cobalt can be sputtered in a DC magnetron sputter reactor of the type illustrated schematically in FIG. 1.
A conventional PVD reactor 10 is illustrated schematically in cross section in FIG. 1, and the illustration is based upon the Endura PVD Reactor available from Applied Materials, Inc. of Santa Clara, Calif. The reactor 10 includes a vacuum chamber 12 sealed through a ceramic isolator 14 to a PVD target 16 composed of the material, usually a metal, to be sputter deposited on a wafer 18 held on a heater pedestal electrode 20 by a wafer clamp 22. Alternatively to the wafer clamp 22, an electrostatic chuck may be incorporated into the pedestal 20. The target material may be aluminum, copper, aluminum, titanium, tantalum, alloys of these metals or with alloying elements of up to a few percentages, or other metals amenable to DC sputtering. A shield 24 held within the chamber protects the chamber wall 12 from the sputtered material and provides the anode grounding plane. A selectable and controllable DC power supply 26 negatively biases the target 16 to about -600VDC with respect to the shield 24. Conventionally, the pedestal 20 and hence the wafer 18 are left electrically floating, but it nonetheless develops some DC self-bias to attract positively charged ions from the plasma.
A first gas source 34 supplies a sputtering working gas, typically argon, to the chamber 12 through a mass flow controller 36. The can working gas can be admitted from various positions with the chamber 12 including from the bottom, as illustrated, with one or more inlet pipes supplying gas at the back of the shield. The gas penetrates the bottom of the shield 24 or through a gap 42 between the wafer clamp 22 and the shield 24 and the pedestal 20. A vacuum system 44 connected to the chamber 12 through a wide pumping port 46 maintains interior of the chamber 12 at a low pressure. Although the base pressure can be held to about 10.sup.-7 Torr or even lower, the conventional pressure of the argon working gas is typically maintained at between about 1 and 1000 mTorr. A computer-based controller 48 controls the reactor including the DC power supply 26 and the mass flow controller 36.
When the argon is admitted into the chamber, the DC voltage applied between the target 16 and the shield 24 ignites the argon into a plasma, and the positively charged argon ions are attracted to the negatively charged target 16. The ions strike the target 16 at a substantial energy and cause target atoms or atomic clusters to be sputtered from the target 16. Some of the target particles strike the wafer 18 and are thereby deposited on it, thereby forming a film of the target material.
To provide efficient sputtering, a magnetron 50 is positioned in back of the target 14. It has opposed magnets 52, 54 coupled by a magnetic yoke 56 producing a magnetic field within the chamber in the neighborhood of the magnets 52, 54. The magnetic field traps electrons and, for charge neutrality, the ion density also increases to form a high-density plasma region 58 within the chamber adjacent to the magnetron 50. To achieve full coverage in sputtering of the target 14, the magnetron 50 is usually rotated about the center 60 of the target 14 by a shaft 62 driven by an unillustrated motor.
Sputtering of the very thin cobalt layer needed for siliciding does not require a high deposition rate nor, as a result, a significantly high plasma density in the region beneath the magnetron. Therefore, the target power may be set to less than 1 kW for a 200 mm wafer, compared to 20 kW or more for some types of aluminum or copper sputtering. For such thin layers required for siliciding, the layer thickness must be carefully controlled so that excessive silicon is not consumed in the siliciding process. Lower power for sputtering thin cobalt layers improves the thickness control.
Sputtering of cobalt, though, presents some fundamentally different problems from the sputtering of aluminum, copper, or titanium. Cobalt is a ferromagnetic material. As a result, the magnetic field produced by the magnetron is at least partially shunted through the cobalt target and does not contribute to the formation of the high-density plasma region. Some diminution of the plasma density because of the reduced magnetic flux beneath the magnetron is not a major problem for cobalt sputtering because the cobalt deposition rate need not be all that high. However, ignition of the plasma with a ferromagnetic target does present a problem.
Plasma ignition can present a significant problem, especially in the geometries representative of a commercially significant plasma reactor. The initial excitation of a plasma requires a high voltage, though with essentially no current, to cause the working gas to be excited into the electrons and positive ions of an electron. This condition must persist for a time period and over a space sufficient to support a low-resistance, essentially neutral plasma between the two electrodes in the case of a capacitively coupled plasma. The maintenance of a plasma requires a feedback condition in which at least as many argon atoms, if argon is the dominant gas, are excited into ions and electrons as are lost. Electron loss to the walls is the usual limiting factor. If too many electrons are lost, the plasma collapses or is never formed.
It has been observed that plasma ignition with a cobalt plasma is very unreliable. Often ignition requires as much time as would be expended in the actual deposition of cobalt and requires several attempts at the ignition sequence.
Hence, it is greatly desired that a means be provided for reliably igniting the plasma for sputtering cobalt and other materials, particularly ferromagnetic materials.